Computed Properties and Aggregate Data

Sometimes you have a computed property whose value depends on the properties of items in an array. For example, you may have an array of todo items, and want to calculate the incomplete todo's based on their isDone property.

To facilitate this, Ember provides the @each key illustrated below:

export default Ember.Component.extend({
  todos: null,

  init() {
    this.set('todos', [
      Ember.Object.create({ isDone: true }),
      Ember.Object.create({ isDone: false }),
      Ember.Object.create({ isDone: true }),
    ]);
  },

  incomplete: Ember.computed('todos.@each.isDone', function() {
    let todos = this.get('todos');
    return todos.filterBy('isDone', false);
  })
});

Here, the dependent key todos.@each.isDone instructs Ember.js to update bindings and fire observers when any of the following events occurs:

1. The isDone property of any of the objects in the todos array changes.
2. An item is added to the todos array.
3. An item is removed from the todos array.
4. The todos property of the component is changed to a different array.

Ember also provides a computed property macro computed.filterBy, which is a shorter way of expressing the above computed property:

export default Ember.Component.extend({
  todos: null,

  init() {
    this.set('todos', [
      Ember.Object.create({ isDone: true }),
      Ember.Object.create({ isDone: false }),
      Ember.Object.create({ isDone: true }),
    ]);
  },

  incomplete: Ember.computed.filterBy('todos', 'isDone', false)
});

In both of the examples above, incomplete is an array containing the single incomplete todo:

import TodoListComponent from 'app/components/todo-list';

let todoListComponent = TodoListComponent.create();
todoListComponent.get('incomplete.length');
// 1

If we change the todo's isDone property, the incomplete property is updated automatically:

let todos = todoListComponent.get('todos');
let todo = todos.objectAt(1);
todo.set('isDone', true);

todoListComponent.get('incomplete.length');
// 0

todo = Ember.Object.create({ isDone: false });
todos.pushObject(todo);

todoListComponent.get('incomplete.length');
// 1

Note that @each only works one level deep. You cannot use nested forms like todos.@each.owner.name or todos.@each.owner.@each.name.

Sometimes you don't care if properties of individual array items change. In this case use the [] key instead of @each. Computed properties dependent on an array using the [] key will only update if items are added to or removed from the array, or if the array property is set to a different array. For example:

export default Ember.Component.extend({
  todos: null,

  init() {
    this.set('todos', [
      Ember.Object.create({ isDone: true }),
      Ember.Object.create({ isDone: false }),
      Ember.Object.create({ isDone: true }),
    ]);
  },

  selectedTodo: null,
  indexOfSelectedTodo: Ember.computed('selectedTodo', 'todos.[]', function() {
    return this.get('todos').indexOf(this.get('selectedTodo'));
  })
});

Here, indexOfSelectedTodo depends on todos.[], so it will update if we add an item to todos, but won't update if the value of isDone on a todo changes.

Several of the Ember.computed macros utilize the [] key to implement common use-cases. For instance, to create a computed property that mapped properties from an array, you could use Ember.computed.map or build the computed property yourself:

const Hamster = Ember.Object.extend({
  excitingChores: Ember.computed('chores.[]', function() {
    return this.get('chores').map(function(chore, index) {
      return `CHORE ${index}: ${chore.toUpperCase()}!`;
    });
  })
});

const hamster = Hamster.create({
  chores: ['clean', 'write more unit tests']
});

hamster.get('excitingChores'); // ['CHORE 1: CLEAN!', 'CHORE 2: WRITE MORE UNIT TESTS!']
hamster.get('chores').pushObject('review code');
hamster.get('excitingChores'); // ['CHORE 1: CLEAN!', 'CHORE 2: WRITE MORE UNIT TESTS!', 'CHORE 3: REVIEW CODE!']

By comparison, using the computed macro abstracts some of this away:

const Hamster = Ember.Object.extend({
  excitingChores: Ember.computed.map('chores', function(chore, index) {
    return `CHORE ${index}: ${chore.toUpperCase()}!`;
  })
});

The computed macros expect you to use an array, so there is no need to use the [] key in these cases. However, building your own custom computed property requires you to tell Ember.js that it is watching for array changes, which is where the [] key comes in handy.